As is well known, the standard specifies the encoded (or compressed) representation of video sequences for digital storage and digital video communications and defines the decoding process. The encoding process is not defined in the standard, and it is essentially a hybrid coding, using time prediction techniques with motion estimation (at the pixel block level) to reduce time redundancy, and two-dimensional transform techniques applied to a current picture or to the significant differences between the current picture and a predicted picture to reduce spatial redundancy. Information on the encoding procedures (e.g. with or without prediction and/or motion compensation), motion information and spatial information are then transmitted after encoding with a variable length code. Decoding entails processing the compressed sequences in successive steps, until the original picture sequence is recovered for its subsequent display. In particular, after decoding the variable length code and re-ordering the transmitted coefficients, the latter are subjected to inverse quantization, and spatial and time redundancies are re-introduced. Greater detail can be found in the standard mentioned above as well as in standard ISO/IEC 11172-2 (ISO/MPEG1) and in the paper "The MPEG video compression algorithm", by D. J. Le Gall, Signal Processing: Picture Communication, Vol. 4, No. 2, pp. 129 et seq.
Several devices for encoding and/or decoding MPEG2 video sequences are commercially available. These devices perform all of the functions of an encoder or decoder in accordance with the standard. In general, however, such devices are not meant for professional use (i.e. for use by a television broadcaster or by producers of video disks or compact disks) but rather for "home" users: hence, they only allow pre-set and limited variations of the encoding parameters, and the quality of the encoded or decoded signals they provide is sufficient only if the signals have not been subjected to particular manipulations (e.g. a succession of encoding and decoding operations, as may occur in a television transmission). In general, it is not possible to manipulate these parameters through a control processing unit. To obtain the high encoding quality required for professional use, the possibility of acting on several encoding and decoding parameters at different stages of their respective processes must be provided for this entails the introduction of arrays of discrete components in the different parts of the equipment, which results in larger equipment size and higher costs.